1. Field of the Invention
The present invention relates to an apparatus for detecting relative displacement information and a drive control apparatus using the same. The present invention can be suitably applied to an encoder for detecting the position, position deviation amount, position deviation direction, speed, acceleration, or the like, of a scale by irradiating a light beam onto a diffraction grating on the scale attached to a relatively moving object, and detecting signal light modulated by the surface wavefront phase intensity obtained therefrom.
2. Related Background Art
Conventionally, an incremental encoder is utilized for the purpose of measuring the deviation information (deviation amount, speed, acceleration, or the like) of an object with high precision.
In a typical conventional incremental encoder, a light beam is irradiated onto an amplitude grating (contrast grating) which is recorded on a relatively moving scale and has a pitch on the order of 100 microns, and the light transmitted through the grating is transmitted through an amplitude grating having a pitch equal to that of the grating on the scale, thereby modulating the light beam so that the transmitted light is maximized when the transmission portions of the two gratings overlap each other, and the transmitted light is minimized when the two gratings overlap each other with a shift corresponding to a 1/2 pitch. In this manner, a change in amount of light for one period upon movement by one pitch of the scale grating is generated, and is detected by a photoelectric element to count the periods, thereby outputting an incremental encoder signal. The measurement resolution equals the grating pitch.
The recent trend is to require a higher resolution of an encoder. However, when an encoder is arranged based on the above-mentioned principle, the grating pattern of light transmitted through the first grating is blurred due to the diffraction phenomenon of light, and it becomes difficult to obtain intensity modulation of light amount when transmitted light is obtained by superposing the second grating. Thus, in order to eliminate the influence of the diffraction phenomenon of light, the first and second gratings approach each other as much as possible.
In order to further increase the resolution of an encoder (for example, grating pitch=10 .mu.m), the distance between the first and second gratings must be reduced to about several 10 .mu.m.
However, since the movement of the relatively moving scale is restricted by stages constituted by various guide mechanisms, the first and second gratings may collide against and damage each other due to errors of the mechanism parts.